That was a question posed to me by a friend who just recently discovered my switch from one who used to have a belief in God to one that has a lack thereof.
They stated "Most people I know, including your truly, were not much interested in closely following the tenants of the faith we were brought up in when we were in the prime of our young adulthood. You on the other hand, were much more zealous back then and now have pretty much left it all behind. Why? Surely it is not because you are not sowing your oats at this time."
I went on to tell him he was correct that I was not going through my second puberty. I told him it was because I wanted to believe what was true regardless of where that led me.
After leaving the Jehovah's Witness religion I tried going to various other denominations. However my exit from the JW's had taught me to be much more skeptical in my thinking and I applied this to these other avenues to the Big Guy in the sky I was then traversing.
After a few years of this it became obvious while these other faith systems might be more palatable and easier to believe, they were just as untrue.
I also used this time to read the bible from cover to cover and I had no choice but to conclude that this piece of literature was the thoughts and musings of bronze-age nomads who were trying to keep their small but growing clan spirits high and more importantly to maintain control. An invisible being in the sky who watched your every move and knew your every thought was the best security system of the day.
In due time it became obvious to me that there was no evidence of a benevolent (or malevolent) entity above who listened to our petitions or exclamations of thankfulness.
Since I was now interested in only believing true things, I was led to the path of agnosticism and atheism.
Can I say for 100% certainty that there is no God or being existing outside of the natural world? Of course the truthful answer has to be "No, I don't"
But I am 100% certain that if such an omniscient and omnipotent entity does exist, it surely was not the one who revealed himself as Yahweh (Jehovah) to Moses and Abraham who later had a child out of wedlock and named him Jesus.